The bandwidth requirement of chip-to-chip interconnections within large computer systems and network switches continues to grow at a very rapid pace. Current electrical interconnection schemes are experiencing bottlenecks due to the physical constraints of large, massively parallel bundles of electrical cables, connector size and/or limits on electrical bandwidth due to power and speed considerations. This has led to the increasing utilization of fiber optic interconnections which are preferred for long haul telecommunications links. Fiber optic interconnections can also be advantageously employed to link computer racks or shelves.
As central processing unit (CPU) clock speed continues to increase, and integrated circuits continue to become increasingly miniaturized, optical connections can be employed on circuit board assemblies to accommodate increasing bandwidth requirements. In addition to conductive circuit traces, optical waveguides have been employed on circuit board assemblies in lieu of discrete optical fibers. It is possible to use batch fabrication methods to apply and pattern polymer materials to form the desired optical waveguides. Thus arrays of optical waveguides can be formed on circuit board assemblies to augment traditional conductive paths. Electrical connections between conductive paths and other electrical devices are typically made with solder or removable connectors that do not require a high degree of mechanical alignment in order to function properly. However, an optoelectronic device, such as vertical cavity surface emitting laser (VCSEL), requires precision alignment with an optical waveguide or another optoelectronic device, such as an avalanche photo-diode (APD), to ensure integrity of the signal transmission with minimal optical losses. For example, optoelectronic devices typically require alignment accuracy on the order of a few microns depending upon the specific optical design and implementation. This is significantly greater alignment accuracy than needed for electrical interconnections. Thus, there is a need for an optical assembly that can simultaneously achieve electrical and optical interconnections with the latter being achieved with a high degree of accuracy.
An alignment maintenance problem that needs to be addressed in an optical assembly that provides simultaneous electrical and optical interconnections arises from the fact that the different materials in the assembly have different coefficients of thermal expansion. Thus it is difficult to maintain alignment of optoelectronic devices within a few microns over a full range of fabrication and operating temperatures. For example if the design of the optical assembly requires an elevated temperature during fabrication, and it thereafter cools to ambient temperature, the optoelectronic device may become misaligned relative to its waveguide. Thus the optical link may not achieve signal transfer integrity during subsequent operation. In addition, as the temperature within a computer or switch cabinet cycles over time during normal operation the optoelectronic device may also become misaligned and therefore fail to correctly perform its intended function.
An optical assembly that can simultaneously achieve electrical and optical interconnections must be designed so that any sensitive components such as lenses are not scratched during handling. Moreover, it should be capable of accommodating arrays of optoelectronic devices.